Power-shifting automatic transmissions of both the planetary type and countershaft type use hydraulically actuated, friction torque devices to effect the selection of sequential drive ranges. Planetary type transmissions use friction torque transfer devices of both the clutch and brake variety. Countershaft type transmissions use friction torque transfer devices of only the clutch variety. The control mechanism which determines the shift sequence and timing for these transmissions can be either hydraulic control valving or the more recently introduced electro-hydraulic control valving. With electro-hydraulic controls, a pre-programmed digital computer is generally provided to determine both the shift schedules and pressure levels of the hydraulic actuating fluid within the transmission. The computer employs a look-up table which has the necessary data to determine the shift points in response to input signals from vehicle parameter detectors such as the vehicle and engine speed sensors, engine torque level sensors, throttle position sensors, and the like.
The computer analyzes the input signals and refers to the look-up table to determine the appropriate ratio interchange. The computer can also provide the necessary control signals to establish the output pressure of the solenoid valve. Generally the solenoid valves are either of the off-on type or the pulse width modulated (PWM) type. With either type, the output signal is delivered to either a valve, which will control the ratio interchange, or to the friction devices directly.
The control devices currently known have a governor and throttle signal to control the ratio interchange. In some instances this signal is combined by the electronics to provide a single electrical output signal which will determine the output pressure of the solenoid control valving. Should the solenoid have a malfunction, the transmission control includes a limp-home feature which causes the transmission to select a fixed gear ratio until proper repairs are undertaken. This feature prevents the driver from being stranded due to an electric or mechanical malfunction of the solenoids.